Noninvasive observations of fluorinated anesthetics in rabbit brain by fluorine-19 nuclear magnetic resonance

Extending the Scope of 19F Hyperpolarization through Signal Amplification by Reversible Exchange in MRI and NMR Spectroscopy

Fluorinated ligands have a variety of uses in chemistry and industry, but it is their medical applications as ¹⁸F-labelled positron emission tomography (PET) tracers where they are most visible. In this work, we illustrate the potential of using ¹⁹F-containing ligands as future magnetic resonance imaging (MRI) contrast agents and as probes in magnetic resonance spectroscopy studies by significantly increasing their magnetic resonance detectability through the signal amplification by reversible exchange (SABRE) hyperpolarization method. We achieve ¹⁹F SABRE polarization in a wide range of molecules, including those essential to medication, and analyze how their steric bulk, the substrate loading, polarization transfer field, pH, and rate of ligand exchange impact the efficiency of SABRE. We conclude by presenting ¹⁹F MRI results in phantoms, which demonstrate that many of these agents show great promise as future ¹⁹F MRI contrast agents for diagnostic investigations.

Temporal accumulation and localization of isoflurane in the C57BL/6 mouse and assessment of its potential contamination in 19 F MRI with perfluoro-crown-ether-labeled cardiac progenitor cells at 9.4 Tesla

Purpose

To assess the uptake, accumulation, temporal stability, and spatial localization of isoflurane (ISO) in the C57BL/6 mouse, and to identify its potential interference with the detection of labeled cardiac progenitor cells using ¹⁹F MRI/MR spectroscopy (MRS).

Materials and Methods

Objectives are demonstrated using (a) in vitro ISO tests, (b) in vivo temporal accumulation/spatial localization C57BL/6 studies (n = 3), and (c) through injections of perfluoro‐crown‐ether (PFCE) labeled cardiac progenitor cells into femoral muscle areas of the murine hindlimb post‐mortem (n = 1) using ¹H/¹⁹F MRI/MRS at 9.4 Tesla. Data were acquired using double‐gated spoiled gradient echo images and pulse‐acquire spectra. For the in vivo study, the temporal stability of ISO resonances was quantified using coefficient of variability (CV) (5 min) estimates.

Results

Two ISO resonances were observed in vivo that correspond to the ‐CF₃ and ‐OCHF₂ moieties. CV values ranged between 3.2 and 6.4% (‐CF₃) and 6.4 and 11.2% (‐OCHF₂). Reductions of the ISO dose (2.0 to 1.7%) at 80 min postinduction had insignificant effects on ISO signals (P = 0.23; P = 0.71). PFCE‐labeled cells exhibited a resonance at −16.25 ppm in vitro that did not overlap with the ISO resonances, a finding that is confirmed with MRS post‐mortem using injected, labeled cells. Based on ¹⁹F MRI, similar in vivo/post‐mortem ISO compartmentalization was also confirmed in peripheral and thoracic skeletal muscles.

Conclusion

Significant ISO accumulation was observed by ¹⁹F MRS in vivo with temporally stable signals over 90 min postinduction. ISO effects on PFCE labels are anticipated to be minimal but may be more prominent for perfluoropolyether or perfluorooctyl bromide labels.

Level of Evidence: 1

Technical Efficacy: Stage 1

J. MAGN. RESON. IMAGING 2017;45:1659–1667

Fluorine-19 MRI Contrast Agents for Cell Tracking and Lung Imaging

Fluorine-19 (¹⁹F)-based contrast agents for magnetic resonance imaging stand to revolutionize imaging-based research and clinical trials in several fields of medical intervention. First, their use in characterizing in vivo cell behavior may help bring cellular therapy closer to clinical acceptance. Second, their use in lung imaging provides novel noninvasive interrogation of the ventilated airspaces without the need for complicated, hard-to-distribute hardware. This article reviews the current state of ¹⁹F-based cell tracking and lung imaging using magnetic resonance imaging and describes the link between the methods across these fields and how they may mutually benefit from solutions to mutual problems encountered when imaging ¹⁹F-containing compounds, as well as hardware and software advancements.

Fluorine (19F) MRS and MRI in biomedicine

Shortly after the introduction of (1)H MRI, fluorinated molecules were tested as MR-detectable tracers or contrast agents. Many fluorinated compounds, which are nontoxic and chemically inert, are now being used in a broad range of biomedical applications, including anesthetics, chemotherapeutic agents, and molecules with high oxygen solubility for respiration and blood substitution. These compounds can be monitored by fluorine ((19)F) MRI and/or MRS, providing a noninvasive means to interrogate associated functions in biological systems. As a result of the lack of endogenous fluorine in living organisms, (19)F MRI of 'hotspots' of targeted fluorinated contrast agents has recently opened up new research avenues in molecular and cellular imaging. This includes the specific targeting and imaging of cellular surface epitopes, as well as MRI cell tracking of endogenous macrophages, injected immune cells and stem cell transplants.

In vivo magnetic resonance imaging to detect biliary excretion of 19F-labeled drug in mice

Isoflurane is an inhaled halogenated hydrocarbon anesthetic commonly used for animal research. In our quest to develop a method for measuring bile acid transport in live animals using (19)F magnetic resonance (MR) imaging, it occurred to us that isoflurane, which contains five fluorines per molecule and is probably widely distributed, would provide an excellent test drug to evaluate the merits of this approach. Experiments in 20- to 28-g male C57BL/6 mice were performed using a horizontal bore scanner with a 30-mm (19)F/(1)H dual-tuned surface coil used to transmit and receive radiofrequency signals at 300.28 MHz for (1)H and 282.55 MHz for (19)F nuclei. Proton MR imaging was used to identify the mouse gallbladder in vivo, which was verified by anatomical dissection. Subsequent experiments in mice inhaling 1.5% isoflurane for 1 to 2 h revealed robust (19)F signals from the gallbladder, verified by overlying (1)H and (19)F signals. No (19)F signal was detected in mice anesthetized with nonhalogenated anesthetics. The presence of isoflurane in gallbladder bile of isoflurane-treated mice was verified using liquid chromatography-mass spectrometry. Gallbladder bile isoflurane content ranged from 3.2 to 4.7 μg. The data presented here provide proof of concept that this novel approach can be used for in vivo measurement of biliary excretion of both existing and novel (19)F-labeled drugs.

In vivo NMR studies of neurodegenerative diseases in transgenic and rodent models

In vivo magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) provide unique quality to attain neurochemical, physiological, anatomical, and functional information non-invasively. These techniques have been increasingly applied to biomedical research and clinical usage in diagnosis and prognosis of diseases. The ability of MRS to detect early yet subtle changes of neurochemicals in vivo permits the use of this technology for the study of cerebral metabolism in physiological and pathological conditions. Recent advances in MR technology have further extended its use to assess the etiology and progression of neurodegeneration. This review focuses on the current technical advances and the applications of MRS and MRI in the study of neurodegenerative disease animal models including amyotrophic lateral sclerosis, Alzheimer's, Huntington's, and Parkinson's diseases. Enhanced MR measurable neurochemical parameters in vivo are described in regard to their importance in neurodegenerative disorders and their investigation into the metabolic alterations accompanying the pathogenesis of neurodegeneration.

In vivo 19F NMR chemical-shift imaging of Ancistrocladus species

19F nuclear magnetic resonance (NMR) imaging and 19F NMR chemical-shift imaging (19F CSI) have been used to localize fluorinated compounds administered to stems of Ancistrocladus heyneanus and A. abbreviatus for the elucidation of biosynthetic pathways in living plants. This first application of 19F CSI on plants proved CSI to be a valuable technique for mapping fluorinated molecules in plants. Exemplarily using trifluoroacetate as a model compound allowed to select appropriate feeding methods and to optimize both concentration and duration of the application to the plant. The time course of the uptake and distribution of trifluoroacetate was monitored by both 19F imaging and 19F CSI. Fluorinated metabolites formed by uptake of 3-fluoro-3-deoxy-D-glucose were detected with 19F CSI.

Uptake and elimination of sevoflurane in rabbit tissues--an in vivo magnetic resonance spectroscopy study

PURPOSE

Previous pharmacokinetic studies of fluorinated anesthetics using 19F-magnetic resonance spectroscopy (19F-MRS) have focused on the brain. Investigation of other tissues would give more precise information about the pharmacokinetics of inhalational anesthetics. In this study we investigated the pharmacokinetics of uptake and elimination of sevoflurane in brain, liver, muscle, venous blood and arterial blood of rabbits.

METHODS

Twenty rabbits were examined by 19F-MRS conducted at 4.7 Tesla using a 1-cm-diameter surface coil for brain (n = 4), liver (n = 5) and muscle (n = 5), and a 1.3-cm-diameter surface coil for arterial (n = 3) and venous (n = 3) blood. Sevoflurane, 4% in oxygen, was administered for 120 min, followed by 120 min elimination.

RESULTS

Both the uptake and elimination kinetics were best fitted by a biexponential curve which was divided into fast and slow components. During the uptake experiment the time required to reach half of the maximum spectroscopic intensity in each tissue was 1.6 min in arterial blood, 4.7 min in liver, 12.2 min in venous blood, 14.4 min in brain and 20.9 min in muscle. During the elimination experiment the time required to reach half maximum intensity was 2.4 min in arterial blood, 6.3 min in liver, 13.4 min in venous blood, 19.6 min in brain and 28.7 min in muscle.

CONCLUSIONS

Sevoflurane uptake or elimination in the tissues examined followed biexponential kinetics. In this rabbit model, sevoflurane uptake and elimination were fastest in arterial blood, followed, in order, by liver, venous blood, brain and muscle.

Changes in plasma cortisol concentrations in bitches in response to different combinations of halothane and butorphanol, with or without ovariohysterectomy

Changes in plasma cortisol concentrations were assessed in bitches in response to nine treatments: control, anaesthesia, analgesia, analgesia followed by anaesthesia, anaesthesia followed by analgesia at intubation, anaesthesia followed by analgesia at extubation, anaesthesia plus surgery, analgesia followed by anaesthesia plus surgery, and anaesthesia plus surgery followed by analgesia. The anaesthetic was halothane, the analgesic was butorphanol (0.4 mg kg(-1)) and the surgery was ovariohysterectomy. Blood samples, for plasma cortisol assays, were taken regularly from before treatment for five hours and then again after 24 hours. A small transient rise in plasma cortisol concentration in the control group was attributed to mild distress associated with novel experiences. A more pronounced and protracted rise in cortisol concentration in the analgesia group was ascribed to a dysphoric state of bitches under the influence of the agonist-antagonist butorphanol. Halothane anaesthesia alone resulted in no change in plasma cortisol concentration. When butorphanol was given after anaesthesia was induced or while the animal was still under the influence of anaesthesia (immediately after tracheal extubation), there was no immediate rise in plasma cortisol concentration and low concentrations were maintained for up to 60 minutes after halothane withdrawal. A marked rise in plasma cortisol concentration, which was sustained above pretreatment values for at least five hours, occurred in all surgery groups. Giving intravenous butorphanol 30 minutes prior to surgery had no effect on the surgically-induced rise in plasma cortisol concentration and no effect on the postsurgical plasma cortisol concentration. In contrast, butorphanol given at extubation did reduce plasma cortisol concentrations during the postsurgical period. These observations did not support the hypothesis that preoperative use of butorphanol would reduce the cortisol response after surgery under halothane anaesthesia.

Isoflurane-mediated inhibition of the constriction of mesenteric capacitance veins and related circulatory responses to acute graded hypoxic hypoxia

We measured the effects of inhaled isoflurane on hypoxemia-induced changes in the diameter of small mesenteric (capacitance-regulating) veins, sympathetic efferent neural activity, heart rate, and arterial blood pressure. Simultaneous changes in these dependent variables were measured in situ in response to 40-s periods of sequentially administered 10%, 5%, 2.5%, and 0% inspired oxygen before, during, and after either 0.75% or 1.5% vol/vol inhaled isoflurane in alpha-chloralose-anesthetized rabbits. Isoflurane inhibited hypoxia-mediated venoconstriction, increases in sympathetic efferent nerve activity, arterial hypertension, and bradycardia. Furthermore, inhibition of diameter, blood pressure, and heart rate responses persisted after washout of isoflurane. Differences in the attenuation of these respective hypoxia-mediated responses were minimal between the two concentrations of inhaled isoflurane. These results further demonstrate that isoflurane alters the ability to produce cardiovascular adjustments to circulatory stress, including changes in vascular capacitance, which is a major regulatory mechanism.

In vivo 19F one-dimensional chemical shift imaging study of isoflurane uptake in rabbit brain

19F one-dimensional chemical shift imaging and one-pulse techniques were used to follow the uptake of isoflurane in rabbit brain in vivo. Brain isoflurane appeared mainly in two adjacent slices of cerebral cortex. Both slices showed increase in anesthetic content at similar rates. A biphasic first-order uptake pattern with an initial fast component and a slower rate for the duration of the uptake was observed by the one-pulse technique. Only the slower kinetic was followed by chemical shift imaging due to constraints on temporal resolution. Anesthetic levels in arterial blood reached equilibrium significantly earlier than in brain, suggesting a 'restricted' diffusion into brain tissue for this agent.

In vivo 19F NMR studies of hyperthermia: hydrophobic environments probed by halothane

The steady-state distribution of the general anesthetic halothane in different rat tissues, including a renal adenocarcinoma with and without hyperthermia treatment, has been evaluated by in vivo 19F NMR spectroscopy. The 19F spectra of halothane (which is a hydrophobic probe) from within tissue show differences in the partitioning between normal rat tissues and adenocarcinoma. Muscle, as a control tissue, exhibits a single large resonance around 0 ppm. However, the adenocarcinoma exhibits two slow-exchanging resonances separated by 0.3 ppm with the one at the more hydrophobic chemical shift being more sensitive to hyperthermia treatment. The results from this tumor model suggest that 19F NMR spectroscopy may be useful first in detecting a change in hydrophobic environments using a lipophilic probe such as halothane, and secondly in monitoring the effects of hyperthermia, a treatment whose effectiveness may involve changes at the level of the plasma membrane. Under conditions of continuous delivery, a resonance which is not detected in the spectra of halothane in excised tissue appears 5 ppm downfield from the resonance for halothane localized in tissues. A rotating frame experiment is used to show that this resonance is derived from anesthetic absorbed on the tissue surface.

In vivo monitoring of changes in 5-fluorouracil metabolism induced by methotrexate measured by 19F NMR spectroscopy

We have used in vivo 19F NMR spectroscopy to study the metabolism of 5-fluorouracil (FUra) in tumors with and without pretreatment with methotrexate (MTX). Using the CD8F1 murine mammary tumor as an in vivo model, we observed signals from FUra, alpha-fluoro-beta-alanine (F beta ALA), alpha-fluoro-beta-ureidopropionic acid (FUPA), and 5-fluorouracil-nucleotides (FUN) after intravenous or intraperitoneal injection of 150 mg/kg FUra. Formation of FUN was increased about 1.7-fold in CD8F1 tumor with methotrexate pretreatment as determined by acid extraction and HPLC analysis. A comparison of in vivo NMR spectra from FUra and sequential MTX + FUra-treated tumors showed a significantly higher ratio of the FUN signal to the initial total 19F signal in the MTX + FUra-treated tumors (p less than 0.001) for animals receiving FUra either intravenously or intraperitoneally. In addition, tumors treated with MTX + FUra had significantly longer time durations during which FUN was detected, independent of the mode of administration. These experiments indicate that in vivo 19F NMR spectroscopy can be used to noninvasively monitor alterations of 5-fluorouracil metabolism that occur with administration of modulating agents such as methotrexate. Further studies, in both murine tumor models and patients, are indicated to determine if these results can be correlated with tumor response.

Regional pharmacokinetics. II. Experimental methods

Regional pharmacokinetics is the study of the drug concentrations in specific regions of the body. It allows greater insight into the mechanisms of drug disposition than the study of systemic blood concentrations. Experimental methods in regional pharmacokinetics and their applications and limitations are reviewed. Post-mortem tissue biopsies give the drug concentrations in highly specific regions of the body, but require a large number of animals. Serial tissue biopsies yield the time-course of drug concentrations in individual animals, but have limited applications. Regional blood sampling in vivo requires catheterization of blood vessels and a measure of regional blood flow, but allows repeated measurements of the time-course of regional drug concentrations in an individual. In contrast, artificially perfused regions allow greater control of perfusate flow and composition, but are less representative of the in vivo situation. These factors can be retained in some animals by surgically transplanting organs to another location to increase access. Tissues slices and cell cultures can examine drug uptake in the absence of perfusion, and tissue homogenates can be used to study the in vitro rates of drug metabolism and tissue drug binding.

Noninvasive in vivo detection of a fluorinated neuroleptic in the human brain by 19F nuclear magnetic resonance spectroscopy

A fluorinated long-acting neuroleptic (fluphenazine decanoate) was detected in the human brain by fluorine nuclear magnetic resonance spectroscopy (MRS). The MRS system used had a 3 Tesla magnetic field, corresponding to a frequency of 118 MHz for the fluorine nucleus (Bruker Medspec 3 Tesla, 60-cm magnet bore width). The spectra were obtained with a surface coil having a 10-cm diameter, which was centered alternatively on the frontal lobe and basal ganglia, and on the occipital lobe. The impregnation and elimination kinetics of fluphenazine were monitored during three sessions (4 hours, 5 days, and 11 days after injection of the compound). This technique can be usefully applied to the in vivo study of the pharmacokinetics and site of action of psychiatric compounds in man.

The application of nuclear magnetic resonance spectroscopy to drug metabolism studies

1. The applications of n.m.r. spectroscopy studies to drug metabolism, both in vitro (biofluids, isolated cells, excised tissue samples, isolated organs) and in vivo (animals, humans) are reviewed. 2. N.m.r. is a relatively insensitive technique, but it has the great advantage of being non-invasive and non-destructive, i.e. it allows a direct study of intact biological samples. 3. The majority of studies examined deal with 19F-n.m.r. spectroscopy mainly because of the favourable n.m.r. characteristics of this nucleus, and the low level of endogenous fluorine which gives no detectable 19F signal. However, the potential utility of 1H-31P- and 13C-n.m.r. is also emphasized.

Isoflurane-induced impairment of synaptic transmission in hippocampal neurons

The effects of anaesthetic applications of isoflurane were studied using intracellular recording techniques in 82 CA1 neurons of in vitro hippocampal slice preparations (guinea pigs). Various parameters of their excitabilities such as membrane electrical properties, action potentials evoked by intracellular current pulse injections and spike afterhyperpolarizations, as well as synaptic potentials evoked by electrical stimulation of stratum radiatum, were determined during bath perfusion of clinical concentrations of isoflurane which were measured with 19fluorine-nuclear magnetic resonance techniques. The vaporizer settings of 1-4% isoflurane corresponded to concentrations of 100 microM to 500 microM. Isoflurane applications did not produce consistent effects on the resting potentials or passive membrane properties. However, when spike-evoked synaptic activity was blocked by tetrodotoxin, isoflurane application induced a hyperpolarization (3-5 mV) without greatly affecting input conductance and the slopes of current-voltage relations. The threshold, amplitude and duration of single or multiple spikes evoked by current injections also were not greatly altered by isoflurane applications. However, marked reductions were observed in the amplitudes of the long-lasting hyperpolarizations following an evoked train of a constant number (4 or 5) of spikes. The amplitudes of excitatory postsynaptic potentials evoked by electrical stimulation of stratum radiatum were diminished markedly during isoflurane applications; these effects, like those on the afterhyperpolarizations, were closely dependent on the dose and duration of the application. Low doses (less than 1%) of isoflurane reduced the amplitudes of inhibitory postsynaptic potentials whereas higher doses (1-4%) increased their amplitudes and durations. The effects on afterhyperpolarizations and synaptic potentials could not be attributed to anaesthetic related changes in the resting potentials of the neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo measurement of lithium in humans by nuclear magnetic resonance spectroscopy

In vivo lithium-7 NMR spectroscopy was used to measure muscle and brain lithium levels following the administration of both single and multiple doses of lithium carbonate to normal volunteers. This is the first report of the noninvasive measurement of tissue lithium levels in humans. The results suggest that the relatively slow accumulation of lithium in the brain may be responsible for the delay in therapeutic response that is frequently observed after the initiation of therapy. Further application of this technique should provide a wealth of information on the pharmacokinetics and tissue distribution of lithium in humans.

Equilibration of halothane with brain tissue in vitro: comparison to brain concentrations during anesthesia

A method was devised for reproducing anesthetic concentrations of halothane in slice and membrane preparations of rat brain in vitro. Rats were anesthetized with varying concentrations of halothane, responsiveness was tested, and brain halothane content was determined by heptane extraction and gas chromatography. The inspired concentration of halothane at which half of all animals were unresponsive was 1.05%. At 1.25% halothane, all animals were unresponsive and brain halothane was determined to be 41 +/- 1.3 nmol/mg lipid. No significant differences in halothane concentration between whole brain and a variety of brain regions were detected. To obtain similar concentrations in vitro, membranes or slices of cerebral cortex were incubated in Krebs-Ringer bicarbonate buffer (KRB) that had been preequilibrated with anesthetic. Halothane equilibrated rapidly with the buffer and the tissues. The partition coefficient between gas and KRB was found to be 0.78, and between brain slices and KRB approximately 12. Slightly lower gas concentrations were necessary in vitro than in vivo to obtain the same tissue levels of anesthetic. Using this method, it was shown that there was no effect of anesthetic concentrations of halothane on the uptake of [3H]norepinephrine or [3H]choline into slices of rat cerebral cortex.

In vivo 19F-NMR study of halothane distribution in brain

Halothane distribution and elimination from rabbit brain was studied in vivo using 19F-NMR spectroscopy. Two exponential decay functions for the anesthetic were observed in the clearance curve. They are assigned to halothane in brain held in two distinct chemical environments characterized by different chemical shifts, and half-lives (25 and 320 min). A nonvolatile halothane metabolite with a half-life of several days was found to be present in rabbit brains. The in vivo results were corroborated by ex vivo experiments on excised brain tissue. Halothane was distributed in all of the major cell subfractions, whereas the metabolite was present predominantly in the cytoplasm.

Proton decoupled fluorine nuclear magnetic resonance spectroscopy in situ

The efficacy of proton decoupling for enhancing the 19F nuclear magnetic resonance (NMR) signal-to-noise ratio and spectral resolution in the intact subject is demonstrated. A geometrically orthogonal cross-coil antenna configuration (Helmholtz pair, surface coil) is employed to provide 40 dB of isolation between the 19F observe and 1H decouple frequencies of 188 and 200 MHz, respectively. Further isolation is achieved through the use of high-quality notch filters on both observation and decoupling channels. Application of 19F-(1H) NMR spectroscopy to the study of 2-fluoro-2-deoxy-D-glucose metabolism in cerebral tissue in situ is presented. Significant improvements in sensitivity and resolution are obtained and result from both a collapse of the JFH multiple structure and a substantial positive nuclear Overhauser effect (NOE). To our knowledge, this is the first such demonstration of 1H decoupling in conjunction with 19F observation for study of the metabolism of a fluorinated compound in the living subject.

31P nuclear magnetic resonance in vivo spectroscopy of the metabolic changes induced in the awake rat brain during KCN intoxication and its reversal by hydroxocobalamine

Radiofrequency surface coils were chronically implanted in rats, which were subsequently subjected to 31P nuclear magnetic resonance (NMR) investigations at 4.7 T. The implanted coil allowed study of the animals without need for anesthesia, which is a prerequisite for studies of normal brain metabolism. The animals may be kept in the NMR probe for several hours. During subsequent experiments, they may be placed in the same position, therefore allowing follow-up studies for periods as long as 2 months. This method has been used in the study of sublethal KCN intoxication. KCN, a cytochrome c oxidase inhibitor, induces a blockade of cell respiratory processes, which is reflected, in a dose-dependent manner, by a decrease in phosphocreatine content and pH and an increase in inorganic phosphate content, whereas ATP levels remain constant until high doses of KCN (6 mg/kg i.p.) are reached. 31P NMR allows the time course of these metabolic changes to be followed. For high KCN doses, a new peak, termed X, is observed, which is interpreted as being due to a pool of inorganic phosphate at very low pH (5.65), corresponding to a subset of cells that did not survive KCN injury. Hydroxocobalamine, a specific antidote of KCN, suppresses the metabolic changes due to 6 mg/kg of KCN.

Fluorine-19 NMR studies of tumor-bearing rats treated with difluoromethylornithine

Copenhagen rats with implanted metastatic prostate carcinomas have been treated with the drug alpha-difluoromethylornithine (DFMO), an inhibitor of the enzyme ornithine decarboxylase. In vivo and in vitro 19F NMR observations were then carried out on a variety of organs and tissues. The distribution of the drug strongly favored tumor over surrounding muscle. DFMO, which gives a spectrum similar to that of the pH indicator difluoromethylalanine, has potential for determination of in vivo pH. However, in contrast to the alanine analog, DFMO exhibits a considerably smaller shift dependence in response to pH changes.

Spin-echo fluorine magnetic resonance imaging at 2 T: in vivo spatial distribution of halothane in the rabbit head

Spin-echo 19F magnetic resonance imaging was performed at 2.0 T to explore the in vivo spatial distribution of halothane in the rabbit head. Because the halothane concentration is low in vivo, and because the measured relaxation times of the 19F resonance peak for halothane were T1 approximately equal to 1.0 sec and T2 approximately equal to 3.5-65 msec, 1-3-h imaging times were required (TR = 1 sec, TE = 9 msec) in order to obtain adequate images with a 64 X 256 raw data matrix and a 20-mm slice thickness. With this technique, halothane was primarily detected in lipophilic regions of the rabbit head, but little or no halothane was observed in brain tissue. Because T2 was shorter in brain tissue than in surrounding fat, a shorter TE than we could obtain is needed for optimal spin-echo imaging of brain halothane.

Fluorine-19 NMR spectroscopic studies of the metabolism of 5-fluorouracil in the liver of patients undergoing chemotherapy

Fluorine-19 nuclear magnetic resonance allows direct observation of fluorinated drugs and their metabolites in the human body without background signal from the tissue. A well-known fluorinated chemotherapeutic drug, 5-fluorouracil, and its metabolites were observed noninvasively in the liver of three patients undergoing cancer chemotherapy. Spectra were obtained at 1.5 T with a surface coil centered over the right lobe of the patient's liver. Administration of 1.5 gm of 5-fluorouracil was done after positioning in the magnet. Serial spectra, collected over a 2-h period, revealed both the nature of the metabolites present in the liver, and the time course of each patient's metabolism. These observations represent the first noninvasive NMR study of drugs in human patients and show the feasibility of using in vivo F-19 NMR spectroscopy for human studies of fluorinated compounds.

Fluorinated psychopharmacological agents: noninvasive observation by fluorine-19 nuclear magnetic resonance

Fluorinated psychopharmacological agents were measured with fluorine-19 nuclear magnetic resonance spectroscopy in the brain of intact rats that had been treated with fluphenazine. These in vivo experiments were compared to in vitro measurements of fluphenazine-treated rats. A high-field shift was observed in both in vivo and in vitro measurements. On the basis of the in vitro measurements, fluphenazine concentration in the brains of treated rats was estimated. Our observations demonstrate the feasibility of this technique for determining fluorinated neuroleptics in live mammals.

Nuclear magnetic resonance. Its implications for the anaesthetist

The theory and practice of clinical nuclear magnetic resonance (NMR) imaging is reviewed. Problems which the anaesthetist will encounter are considered, and recommendations are proposed. Possible uses of NMR in anaesthesia are discussed.